U.S. patent number 3,795,043 [Application Number 05/196,442] was granted by the patent office on 1974-03-05 for method for obtaining beam lead connections for integrated circuits.
This patent grant is currently assigned to Honeywell Information Systems Italia. Invention is credited to Franco Forlani.
United States Patent |
3,795,043 |
Forlani |
March 5, 1974 |
METHOD FOR OBTAINING BEAM LEAD CONNECTIONS FOR INTEGRATED
CIRCUITS
Abstract
A process for obtaining beam leads for connecting integrated
circuit chips to an external circuit substrate, wherein aluminum
beam leads are formed and directly and simultaneously bonded to
aluminum pads on the chips while continuously being supported.
Inventors: |
Forlani; Franco (Monza,
IT) |
Assignee: |
Honeywell Information Systems
Italia (Caluso, IT)
|
Family
ID: |
26328921 |
Appl.
No.: |
05/196,442 |
Filed: |
November 2, 1971 |
Foreign Application Priority Data
|
|
|
|
|
Nov 5, 1970 [IT] |
|
|
31333/70 |
|
Current U.S.
Class: |
29/593;
257/E21.509; 174/551; 174/557; 29/827; 438/123; 438/15; 438/111;
257/E23.066; 257/E23.055 |
Current CPC
Class: |
H01L
23/49861 (20130101); H01L 23/49572 (20130101); H01L
24/86 (20130101); H01L 24/50 (20130101); H01L
2924/01005 (20130101); H01L 2924/01082 (20130101); Y10T
29/49135 (20150115); H01L 2924/01019 (20130101); H01L
2924/01039 (20130101); H01L 2924/01013 (20130101); H01L
2924/10253 (20130101); H01L 2924/01074 (20130101); H01L
2924/10253 (20130101); Y10T 29/49144 (20150115); H01L
2924/01079 (20130101); H01L 2924/15787 (20130101); H01L
2924/15787 (20130101); Y10T 29/49004 (20150115); H01L
2924/01006 (20130101); H01L 2924/014 (20130101); H01L
2924/01015 (20130101); Y10T 29/49121 (20150115); H01L
2924/01327 (20130101); H01L 2924/14 (20130101); H01L
2924/01033 (20130101); H01L 2924/00 (20130101); H01L
2924/00 (20130101) |
Current International
Class: |
H01L
21/60 (20060101); H01L 23/498 (20060101); H01L
23/48 (20060101); H01L 23/495 (20060101); H01L
21/02 (20060101); B01j 017/00 () |
Field of
Search: |
;29/574,578,576S,589,577 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Lake; Roy
Assistant Examiner: Tupman; W. C.
Attorney, Agent or Firm: White; William F. Reiling; Ronald
T.
Claims
I claim:
1. A continuous process for obtaining beam leads for a plurality of
integrated circuits, comprising the following steps:
applying a photosensitive plastic film on one surface of a metallic
ribbon to thereby form a photosensitive plastic film substrate for
the metallic ribbon;
applying a layer of photoresist on the other surface of said
metallic ribbon to provide an elongated sandwich ribbon;
moving the sandwich ribbon to an exposure station;
exposing said photoresist at said exposure station to radiation
through a first mask;
exposing said photosensitive plastic film substrate at said
exposure station to radiation through a second mask;
moving the exposed sandwich ribbon to a first tank for developing
said photoresist layer and developing said layer in said first tank
to obtain protected regions which represent the pattern of said
beam leads;
etching said sandwich ribbon to remove the unprotected regions of
said metallic ribbon so as to provide said pattern of beam
leads;
moving said sandwich ribbon to a second tank for developing said
photosensitive plastic film substrate and developing said film
substrate in said second tank to obtain openings in said film
substrate which extend through said plastic film substrate to said
pattern to beam leads;
moving said sandwich ribbon to a bonding station and bonding said
beam leads to integrated circuit semiconductor chips positioned
within said openings at said bonding station;
moving said sandwich ribbon to a test station for testing the
integrated circuits on said chips; and
moving said sandwich ribbon to a selecting station and selecting
the integrated circuits from said sandwich ribbon.
2. A process for providing integrated circuit chips with individual
beam leads comprising the following steps:
applying a photosensitive plastic film on one surface of a metallic
sheet to thereby form a photosensitive plastic film substrate;
applying a layer of photoresist on the other surface of said
metallic sheet;
exposing said photoresist layer to radiation through a suitable
masking means;
developing the exposed photoresist layer to obtain protected
regions of said metallic sheet which represent the pattern of said
beam leads;
chemically etching said metallic sheet to remove the unprotected
regions thereof so as to provide said beam leads;
exposing said photosensitive plastic film substrate to radiation
through a suitable mask;
developing and etching the exposed plastic film substrate to
thereby obtain a plastic frame substrate with an opening therein
which extends through said plastic film substrate to said pattern
of beam leads;
positioning a semiconductor chip provided with integrated circuits
in said opening of the plastic frame substrate so as to correspond
with said beam leads; and
bonding said beam leads to appropriate locations on the integrated
circuit provided on said semiconductor chip.
3. The process of claim 2, comprising the following additional
step: electrically testing the integrity of the bonds and the
operating quality of said integrated circuit.
4. A process for providing integrated circuit chips with individual
beam leads employing a laminated structure comprising a substrate
layer of photosensitive plastic and a metallic sheet, comprising
the steps of:
applying a layer of photoresist material to the external surface of
said metallic sheet;
exposing said photoresist layer to a radiation image of said beam
leads;
developing the exposed photoresist layer to obtain protective
regions with the pattern of said beam leads;
etching away those portions of said metallic sheet which are not
protected by said protective regions so as to obtain said beam
leads;
exposing said photosensitive plastic substrate layer to a radiation
image of an area into which said beam leads extend;
developing and etching the exposed plastic substrate layer to
provide an opening which extends through said plastic layer
substrate in said area into which the ends of said beam leads
project;
positioning an integrated circuit chip within said opening which
extends through said plastic layer substrate with connection so as
to register connection pads on said chip with ends of said beam
leads which extend into said opening; and
bonding said pads to the corresponding beam leads.
5. The process of claim 2 further comprising the step of:
separating said beam leads from said plastic frame substrates so as
to thereby obtain a semiconductor chip with accurately positioned
and bonded beam leads.
6. The process of claim 4 further comprising the step of:
separating said beam leads which have been bonded to the conection
pads on said chip from said plastic substrate layer so as to
thereby obtain a semiconductor chip having beam leads which are
accurately bonded to the connection pads of said chip.
7. The process of claim 4, wherein said metallic sheet and said
connection pads are of aluminum.
Description
BACKGROUND OF THE INVENTION
The instant invention relates to production technology for
semiconductor integrated circuits and, more particularly, to the
technology of interconnecting such circuits.
The type of connecting lead preferable for integrated circuits is
known as the beam lead and is well known in the art.
In beam lead technology the semiconductor chip, on which an
integrated circuit is fabricated by deposition and diffusion, is
connected by relatively thick leads, partially projecting beyond
the chip edge, to an external circuit, which may comprise a ceramic
support bearing conductors for connecting to the pins of an
integrated circuit package. Because they are relatively thick, for
example 10 or 20 microns, these leads have sufficient strength for
rigidly positioning the chip with respect to the ceramic support.
Thus, beam leads provide connection means stronger and more
reliable than the extremely thin wires used in other integrated
circuit connection techniques. However, in fabricating beam leads
by methods known heretofore, a number of difficulties and
disadvantages are encountered.
For example, electrolytic accretion methods usually have been
employed to obtain the required thickness, because it has been too
difficult and costly to obtain such thickness by means of
deposition or sputtering processes.
This method limits the choice of the material of the beam leads to
the metals which may be electrolytically deposited, such as gold,
thereby excluding aluminum. Nevertheless, aluminum is usually used
for internal circuit connections on the chip.
Aluminum exhibits excellent adhesion to silicon and silicon dioxide
while not modifying their physical characteristics, not forming
intermetallic compounds, and making optimum electrical contact with
silicon. On the other hand, gold does not insure adequate
mechanical adhesion to silicon dioxide. Therefore, aluminum is
preferred as a conducting metal for internal circuit connections on
the chip and gold has been used for connecting aluminum pads
arranged along the chip edges to external circuits. However, gold
and aluminum in the presence of silicon form an intermetallic
compound known as "purple plague", which impairs the physical,
electrical and mechanical continuity of the junction. Appropriate
measures must be taken to obviate such disadvantages.
The techniques and expediences resorted to for ensuring the
adhesion of the gold conductors to the silicon dioxide layer that
usually overlies a semiconductor chip and for enabling the bonding
together of gold and aluminum considerably limit the scope and
advantage of applying this technology, and increase its cost.
An example of such expediencies is to fabricate beam leads having a
layered structure of different materials, which is described in the
U.S. Pat. No. 3,555,365 issued Jan. 12, 1971, for INTEGRATED
CIRCUIT MATRIX HAVING PARALLEL CIRCUIT STRIPS and assigned to the
assignee of this application.
The techniques employed in these prior art processes also require
that during deposition the chip have dimensions to entirely
encompass the beam lead conductors. Only after their fabrication is
the peripheral portion of the chip chemically etched away to allow
the beam leads to effectively project beyond the chip. This
requires further operating steps and leads to a waste of costly
materials, thereby substantially impairing the economy of the
process.
Accordingly, it is the object of the present invention to provide a
process for obtaining aluminum beam lead conductors which may be
directly and simultaneously bonded to aluminum pads provided on a
chip. It is also an object of the invention to insure the permanent
positioning on the chip and and the shape of such beam leads during
the steps required for their fabrication and bonding to the pads,
and if required, the electrical testing operations.
SUMMARY OF THE INVENTION
The process of the invention is based on first creating the beam
leads by photoetching them out of aluminum foil of proper
thickness, while supporting the beam leads with an underlying
substrate comprising a film of photosensitive plastic material of
suitable mechanical strength. Next, an opening is cut in the film
to provide precise positioning of a semiconductor chip therein, so
that the terminal pads of the chip coincide with the free ends of
the beam leads. The beam leads may be simultaneously bonded to the
corresponding pads of a chip positioned in the opening, for
instance by ultrasonic bonding. The beam lead conductors may
continue to be mutually insulated and held in place by the plastic
underlying film during subsequent operations for electrically
testing the chip, if desired. The film is removed only when the
external ends of the beam lead connectors are bonded to external
conductors, such as those borne by a surrounding ceramic
support.
BRIEF DESCRIPTION OF THE DRAWING
The invention will be described with reference to the accompanying
drawings, wherein:
FIG. 1 is a perspective view of an integrated circuit connected to
a surrounding ceramic support, shown in part, by means of beam lead
connectors;
FIG. 2 is a schematic cross-sectional view showing respectively in
its left and right portions the structure of a photosensitive
plastic film known by the trade name of Riston and the layered
structure of a metallic foil to which the Riston film is applied in
a first step of the process of the invention;
FIG. 3 is a cross-sectional view showing the layered structure from
which the beam lead conductors are obtained in a second step of the
process;
FIG. 4 is a perspective view of a third step of the process,
wherein the structure of FIG. 3 is masked and exposed;
FIG. 5 is a perspective view of the product obtained in a fourth
step of the process;
FIG. 6 is a perspective view of the product obtained in a fifth
step of the process;
FIG. 7 is a schematic diagram illustrating how a continuous
production process may be implemented in accordance with the
instant invention; and
FIG. 8 is a perspective view showing a portion of a product of the
continuous production process of FIG. 7.
DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 1 shows a silicon chip 1 partially overlaid by an insulating
layer 2 of silicon dioxide. By known methods an integrated circuit,
not shown, contained in the area 3 bounded by the dashed line is
fabricated on chip 1. The input and output conductors of the
integrated circuit terminate at aluminum pads 4 and 5, arranged
along the edges of chip 1. In FIG. 1, five beam leads 6 are shown
bonded to corresponding pads 5 along an upper horizontal edge of
chip 1. It is understood that other beam leads, not shown for
clarity, are also bonded to pads 4. Reference numeral 7 indicates
part of a plate, usually made of a ceramic material, which
surrounds chip 1. Leads 8 are deposited on plate 7. The external
larger ends of beam leads 6 are bonded to leads 8, thereby
providing connection of beam leads 6 to the pins of the package
containing the integrated circuit, for example, or to other beam
leads coupled to other chips mounted on plate 7.
The process of the invention is directed to obtaining a silicon
chip with beam leads simultaneously bonded to respective pads
thereof, and in some cases already electrically tested so as to be
ready for connection to an external plate, such process is based on
the use of a photosensitive plastic film of relatively high
mechanical resistance capable of supporting the beam leads.
A plastic film of this type is marketed by the E. I. duPont de
Nemours Company under the trade name of Riston. As shown in the
left portion of FIG. 2, Riston film comprises a thin film, or
layer, 10 of solid plastic photosensitive material. Riston film 10
is shown sandwiched between two transparent films, a polyethylene
film 12 and a polyester film 13 (for example, the product known by
the trade name Mylar). Riston film 10 has good mechanical strength,
exhibits sufficient elasticity to adhere to plane or slightly
curved surfaces, and may be "photopolymerized" by exposure to
light, thereby changing its characteristics.
By selective exposure, using masking means, the photosensitive
materials are selectively modified by polymerization. The unexposed
portions of the photosensitive materials, which were not
polymerized, may be etched away by suitable etchants, whereas the
exposed portions, which were polymerized, remain and retain the
above-mentioned mechanical characteristics. As with other
photosensitive substances, termed "photoresists," used in
photoetching processes, the remaining photosensitive material may
be subsequently removed by the use of proper solvents called
"strippers." It is to be emphasized that plastic photosensitive
materials such as Riston differ from the common photoresists by
being solid in form, thereby providing a good mechanical support
for thin and weak mechanical members adhering thereto, such as beam
leads.
In accordance with the invention beam leads are fabricated and
applied to semiconductor chips by the following described
process.
Polyethylene film 12 is removed from Riston film 10 and then the
photosensitive Riston film 10, covered by mylar film 13, is applied
to adhere to the lower surface of a sheet, foil, or ribbon 11 of
aluminum, which has a thickness of 20 microns, for example. At this
time the structure shown in the right portion of FIG. 2 is
obtained.
A photosensitive paint 14, such as a photoresist, is applied by
known means to the upper surface of aluminum sheet 11, thereby
obtaining the structure shown in FIG. 3. Photoresist 14 may be, for
example, of the positive type.
A photographic mask 15, in which the pattern of the beam leads is
defined by opaque regions, is then applied to the prepared
structure of FIG. 3 and photoresist 14 is exposed through mask 15
to a suitable radiation source 16 (FIG. 4). After such exposure,
the photoresist is developed and etched, whereupon a protective
layer having the same pattern as the desired beam leads remains on
aluminum sheet 11, the other photoresist being removed.
In a succeeding step, aluminum sheet 11 is chemically etched,
thereby removing the aluminum material from the regions not
protected by the photoresist. Thus, beam leads are "carved" out of
the aluminum sheet to their precise shapes and in their respective
required positions. These beam leads adhere by their lower surfaces
to the Riston layer 10. Layer 10 has not been affected by the
above-mentioned operations, because during the preceding exposure
layer 10 was protected from the illuminating radiation by aluminum
sheet 11, and during the preceding development and etching of the
photoresist on the upper face, layer 10 is protected both by
aluminum sheet 11 on one face and by mylar film 13 on the other
face. It is to be stressed that the substances used for developing
the upper face photoresist and the etchant used for removing the
aluminum in the non-protected areas are selected so as to have no
effect on the Riston layer. The structure obtained after this
latter step in the process is shown in FIG. 5.
At this point a bath of a "stripper" solvent is used for removing
the photoresist layer remaining on the upper surface of the beam
leads. This bath must be selective and not affect the Riston film.
For example, acetone is a stripper commonly used for dissolving the
available photoresists and does not affect Riston.
Next, if convenient, although not necessary, the protecting mylar
film 13 is removed from the lower surface of Riston layer 10. Then
the Riston layer is exposed to a radiation source through an
appropriate mask, followed by a development and etching process to
obtain an opening of size and shape suitable for receiving a chip
over which the beam leads project. The structure obtained after
this step of the process is shown in FIg. 6. The beam leads are
mutually electrically insulated and held in proper spaced-apart
positions by a frame of Riston material, which ensures suitable
mechanical rigidity for the entire structure.
A chip bearing an integrated circuit then may be arranged and
centered in the opening under the beam leads, or, alternatively if
conditions warrant, over the beam leads, so that the inner ends of
the beam leads are superposed precisely opposite to corresponding
pads on the chip. All the beam leads may then be bonded
simultaneously to the corresponding pads by ultrasonic means.
Since both of the members to be bonded together in each instance
are of aluminum, an entirely satisfactory bond is obtained, both
from the point of view of electrical conductivity and from the
point of view of mechanical strength, as is known in the art.
After such bonding, the chip with beam leads attached thereto is
separated from the Riston frame, either by using chemical solvents,
or, preferably, by mechanical punching. The punching operation
employs punch which cuts the beam leads to the required length and
bends them in the vertical plane according to requirements either
for supporting the chip in a suitable package or for bonding the
chip to a ceramic plate provided with electrical circuits.
Accordingly, the process described herein is simple and economical
for obtaining integrated circuit chips provided with aluminum beam
leads. Other advantages are also demonstrated; for example, because
the beam leads are electrically insulated while being rigidly held
in position by the Riston frame, electrical measurements and static
and dynamic tests can be made on the chip to test the circuit
operating characteristics and the quality of the chip and of the
bonded points, without the possibility of damaging the relatively
sturdy mechanical assembly formed by the chip and the beam leads.
To improve the rigidity and strength of the structure, an outer
frame of aluminum may be provided, as shown in FIG. 8.
Following the testing operation, the various chips may be selected
and sent directly to a packaging facility or may be stored for
future use.
The process of the invention also lends itself to a continuous
operation, with all the inherent advantages of economy,
repeatability and reliability. Thus, FIG. 7 represents
schematically a method for carrying out the process of the
invention as a continuous process and demonstrates that the order
in which the different steps are performed may differ from the
described order of the steps, without departing from the spirit and
scope of the invention.
In FIG. 7, an aluminum ribbon 51 and a Riston ribbon 52 are unwound
from respective supply rolls 53 and 54. The polyethylene film 55 is
removed from the Riston layer by a roller 56, whereupon the Riston
layer is heated by a heating member 57 and applied with pressure,
as exerted by rollers 58 and 59, against the lower face of aluminum
ribbon 51 to adhere thereto. The upper face of aluminum ribbon 51
is then uniformly coated with photoresist 62 upon passage of the
laminated product through rollers 60 and 61.
The aluminum and Riston ribbon sandwich thus prepared is driven
incrementally through an exposure station 63; i.e., with rapid
advancement steps alternating with halts. Exposure station 63
comprises an upper mask 64 and a lower mask 65 precisely arranged
in opposed positions and radiation sources 66 and 67, which are
intermittently activated. After exposure, the ribbon sandwich
passes through a photoresist development tank 68, an aluminum
etching tank 69 and a stripper tank 70. Then, the mylar film 71 is
removed, and the ribbon sandwich, with the aluminum ribbon 51
already etched and the Riston film exposed, enters a Riston
development tank 72. After this step the ribbon sandwich may
exhibit the structure shown in FIg. 8, wherein the continuous
aluminum ribbon has major openings 80 in the interior of which beam
leads, supported by the Riston film, are located. The Riston film
has noticeably smaller openings 82 than the openings 80 of the
aluminum ribbon, and an appropriate portion of the beam leads is
cantilevered over openings 82. If required, reference holes 83 may
be provided through both the aluminum and the Riston ribbons. The
ribbon sandwich then enters a positioning and bonding station 73,
wherein integrated circuit chips are bonded in the appropriate
positions to the beam leads (FIG. 7).
After emerging from bonding station 73, the ribbon sandwich may
enter a test station 74, wherein the chips and the beam leads are
submitted to testing. As a result of these tests, the chips may be
either accepted or rejected. The rejected chips may be discarded by
punching out at a selecting station 75.
The continuous ribbon sandwich, carrying the accepted chips, may
then proceed to a packaging station, to a storage facility or to an
assembly line, according to the requirements.
* * * * *